In a mobile (cellular) communications network, (user) communication devices (also known as user equipment (UE), for example mobile telephones) communicate with remote servers or with other communication devices via base stations. In their communication with each other, communication devices and base stations use licensed radio frequencies, which are typically divided into frequency bands and/or time blocks.
In recent times there has been an increasing demand for indoor high-speed mobile communication and the increasing deployment of small-sized, localised, cells, often referred to as ‘femtocells’ or ‘pico-cells’, operated by base stations that use a low transmission power is seen one of the inevitable consequences of this trend. Very small-sized cells, such as femtocells, are generally deployed in a home or small office/home office (SOHO) environment with the intention of enhancing capacity of the wider cellular system (e.g. in the manner of a ‘hotspot’) and/or of compensating for a coverage hole in a larger conventional cell or ‘macro’ cell (e.g. at the cell edge).
In long-term evolution (LTE) or LTE-Advanced, a base station that operates a small cell such as a femtocell or the like is often referred to as a ‘home’ base station, home eNodeB (HeNB), low power node (LPN), pico base station, or femto base station. A base station that operates a larger conventional cell is often referred to, in LTE, as a macro base station or Macro eNodeB (MeNB). For the purposes of this document the terms ‘macro base station’ and ‘home base station’ will generally be used.
Where there is an overlap of coverage between them interference between a small cell and the larger macro cell (or between a small cell other small cells) can be a major problem, especially where there is a dense deployment of small cells. This is because small cells are often closed cells and the home base station operating the small cell often shares a common channel with the base stations, such as the macro base station, operating other cells. The uplink (UL) interference from a small cell to a large cell can be a significant contributor to such interference. Thus, communication throughput in a macro cell can be reduced undesirably.
To help mitigate UL interference, it is known for the home base stations that operate small cells to employ UL power control in which the UL transmit power of mobile terminals and/or other UEs communicating via the small cells (home UEs/‘HUEs’) is controlled adaptively (e.g. in UL data and control channels such as the Physical Uplink Shared Channel, ‘PUSCH’ and Physical Uplink Control Channel, ‘PUCCH’).
In many cases, the parameters of UL power control such as the maximum allowed UE transmit power and/or the target received power (user or cell specific) are optimized on the basis of the path loss from the home base station or from a communication device in the small cell (‘small cell UE’) to the macro base station. The path loss is estimated by the difference between the transmit power of the macro base station and the received power at the home base station or UE. However, in this path loss based method, the UL interference from only a single small cell can be considered. Therefore, as the number of small cells increases, the UL interference is also increased resulting ultimately in a significant degradation of the UL throughput in the macro cell.
More recently, a proposal was made for a ‘centralised’ UL power control method in which the target received power for home base stations is adaptively set based on an aggregated resource usage for all small cells in a macro cell. In this more recent proposal a centralised UL power control algorithm (using a Home eNodeB Management System, ‘HeMS’) an algorithm was defined that used the aggregated UL resource usage (physical resource block, ‘PRB’, usage) of all home base stations in a particular macro cell to reduce interference of the home base stations to a macro base station. Where aggregate resource usage was large, the target received power of all home base stations was reduced by reducing the transmit power of all user equipment within the small cell of the home base stations with the intention of reducing overall interference towards the macro base station. Where aggregate resource usage was small, the target received power of all home base stations was increased by increasing transmit power of all user equipment within the small cell of the home base stations with the intention of improving throughput of home base station users.
In this way, therefore, it was expected that UL interference between the small cells, and the larger macro cell, could effectively be suppressed, regardless of the traffic load in the small cell, while the UL transmit power for the small cells could be maximized.